Some air-conditioning apparatuses, such as multi-air-conditioning apparatuses intended for office buildings, each include a heat source device (an outdoor unit) provided outside the building, and indoor units provided inside the building. A refrigerant that circulates through a refrigerant circuit of such an air-conditioning apparatus transfers its heat to (or receives heat from) air supplied to heat exchangers included in the indoor units, whereby the air is heated or cooled. The heated or cooled air is fed into air-conditioned spaces, whereby the air-conditioned spaces are heated or cooled.
A typical heat-source-side refrigerant to be used in such an air-conditioning apparatus is, for example, a HFC (hydrofluorocarbon)-based refrigerant. A natural refrigerant such as carbon dioxide (CO2) is another alternative proposed as the heat-source-side refrigerant.
There is another air-conditioning apparatus, typically, a chiller system, having a different configuration. In such an air-conditioning apparatus, cooling or heating is performed as follows. Cooling energy or heating energy is generated by a heat source device provided outdoor. Subsequently, a heat medium, such as water or antifreeze, is heated or cooled by a heat exchanger included in an outdoor unit. Then, the heated or cooled heat medium is transported to indoor units, such as fan coil units or panel heaters, provided in air-conditioned spaces (see, for example, Patent Literature 1).
Furthermore, there is a heat-source-side heat exchanger called a heat recovery chiller in which a heat source device and each of indoor units are connected to each other by four water pipes, and water or the like that has been cooled and water or the like that has been heated are both supplied to the indoor units, so that cooling or heating is arbitrarily selectable on each of the indoor units (see, for example, Patent Literature 2).
There is yet another air-conditioning apparatus in which a heat exchanger for a primary refrigerant and a secondary refrigerant is provided near each of indoor units, and the secondary refrigerant is transported to the indoor units (see, for example, Patent Literature 3).
There is yet another air-conditioning apparatus in which an outdoor unit and a branch unit that includes a heat exchanger are connected to each other by two pipes, and a secondary refrigerant is transported to indoor units (see, for example, Patent Literature 4).
There is yet another air-conditioning apparatus in which a relay unit including a heat exchanger for a primary refrigerant and a secondary refrigerant is interposed between an outdoor unit and indoor units, and a predetermined heat medium is transported in separate portions to indoor units, respectively (see, for example, Patent Literature 5).